| 摘要: | 此篇論文主旨為成長並製作利用低壓有機金屬化學氣相沈積法所成長之400 nm氮化銦鎵/氮化鎵多重量子井紫外光發光二極體，並且加以研究並探討所造成其發光效率不高之原因，進而設計並優化400 nm紫外光發光二極體之結構。隨後將紫外光發光二極體結構磊晶成長於不同濕式蝕刻深度與方位安排的條狀圖案化藍寶石基板上，並研究與比較其材料與光學特性。最後，製備一種條狀SiON圖案化藍寶石基板，其具備與藍寶石基板相近之折射率，並將400 nm紫外光發光二極體結構磊晶成長於其上，並比較探討其相關光特性。 首先，在探討400 nm紫外光發光二極體之發光行為特性的研究上，我們比較了470 nm藍光發光二極體元件之外部量子效率隨著輸入電流變化的發光行為，在減少熱效應而影響發光行為的考量下，我們亦使用了脈衝式電流輸入方式。經由比較紫、藍光發光二極體元件隨著電流注入之發光行為，發現造成400 nm紫光二極體發光效率衰減之主要原因為載子的溢流情況嚴重所致；同時在脈衝式高電流注入部分，外部量子效率不管在藍光或紫光發光二極體元件皆維持一個常數，此現象打破了載子在量子井中會呈現飽和狀態之說。再者，我們針對5層量子井載子溢流現象嚴重之紫光二極體結構作改善，以期能增加其內部量子效率；隨著量子井層數由5增加至15層，其輸出功率以及外部量子效率改善幅度約大於三倍，認為增加紫外光發光二極體量子井之層數能有效地增加所注入的電子電洞覆合機率，也因此改善了400 nm紫外光發光二極體元件之發光效率。 接著，獲知真正造成紫外光發光二極體之發光效率不高原因後，我們進而優化二極體之結構；在優化400 nm紫外光發光二極體結構之研究中，發現在氮化鎵緩衝層中加入三層SiNx當作中間層，能有效地降低材料之缺陷密度達10倍之多；另外將微量的鋁加入氮化鎵位障層中，不僅不會造成材料之結晶品質劣化反而提增了量子井對於載子的侷限能力。再者，步階式p型氮化鋁鎵障壁層之應用，也可使得發光元件之發光強度大幅提昇，主要原因為有效地增加電洞濃度以及電洞注入量子井中之效率，使得電子電洞覆合機率提升所致。 在氮化鎵與400 nm紫外光發光二極體結構磊晶成長於不同濕式蝕刻深度與方位安排的條狀圖案化藍寶石基板之研究與比較其材料以及光學特性方面，發現氮化鎵之磊晶材料品質與紫外光發光二極體結構之光激發光特性隨著所蝕刻之圖案化藍寶石基板越深則越佳，而且成長在<1-100>sapphire條紋圖案化基板上之材料品質與二極體輸出光特性皆比成長在<11-20>sapphire條紋圖案化基板上來得優越；根據SEM、TEM、CL與二階段磊晶成長於0.9 μm深<11-20>sapphire、<1-100>sapphire圖案化藍寶石基板推測模型之研究分析與遠場光型(far-field pattern)之量測結果，我們深信接近兩倍寬之側向成長低缺陷區域為造成光輸出功率差異之主要因素。另外，在發光二極體結構磊晶成長於不同濕式蝕刻深度與方位安排的條狀圖案化藍寶石基板之內部量子效率與光取出效率的研究分析方面，精確的內部量子效率值可經由變溫之光激發光量測獲得。在二極體結構成長於<11-20>sapphire條紋且不同蝕刻深度之圖案化基板方面，發現內部量子效率與光取出效率之提升量隨著圖案化基板之蝕刻深度由0增加至0.9 μm時，其值分別為29.78 %與44.9 %，說明了在沿著此一維圖案化基板方向之發光二極體的光輸出效率改善上，主要為光萃取效率改善所致。在另一方面，當內部量子效率與光取出效率之提升量隨著圖案化基板之蝕刻深度由0增加至0.9 μm時，其值分別為83 %與44.9 %，說明了在沿著此一維圖案化基板方向之發光二極體的光輸出效率改善上，主要為內部量子效率改善所致。所以在選擇磊晶成長氮化鎵相關材料於一維條狀圖案化藍寶石基板之方面，具有對稱被蝕刻結構之<1-100>sapphire圖案化藍寶石基板為最佳之選擇。 最後，我們開發與利用電漿輔助化學氣相沈積法成長一層可調整折射率的SiON介電材料於藍寶石基板上，藉由調整化合物中O與N的含量，使其折射率與藍寶石基板相同，藉此取代製程費時、不易且可能造成基板表面損害與污染之乾與濕式蝕刻法，並將氮化鎵與400 nm紫外光發光二極體結構磊晶成長於此1 μm厚之條狀SiON圖案化藍寶石基板上（Stripe-SIONPSS）；比較紫外光發光二極體元件成長於Stripe-SIONPSS與傳統的藍寶石基板上之輸出光功率特性，約有60 % 以上之提升。另外，比較了400 nm紫外光發光二極體結構磊晶成長於條狀SiON圖案化基板與平面藍寶石基板之內部量子效率與光取出效率，發現其改善量分別為26.55 %與26.9 %。因此，此輸出光功率之改善原因歸因於缺陷密度之降低以及光萃取效率提升所致；根據以上的分析結果，條狀SiON圖案化藍寶石基板確實為另一有希望且大有可為成為實現高發光效率發光二極體之磊晶基板製作方式。 This dissertation includes the growth and characterization of InGaN/(Al)GaN multiple-quantum-well ultraviolet light-emitting diode (UV LED) structures grown by metalorganic chemical vapor deposition. The main work can be divided into the following four parts. First, a series of measurements have conducted to investigate the luminescence efficiency behaviors of 400 nm UV LEDs with 5 In0.06Ga0.94N/GaN quantum wells. The emission luminescence efficiency of In0.06Ga0.94N/GaN quantum well near UV LED structures were studied for the purpose to clarify the dominant factor to cause low efficiency of UV LEDs and compared with 470 nm blue LED structures with same periods of quantum well in the active region. It is shown that the behavior of luminescence efficiency at low current density for UV and blue LEDs is different due to the different emission mechanisms in the quantum well. Based on their injection current-dependent characteristics under dc and pulsed operation, it can be concluded that carrier overflow is the dominant factor that affects the quantum efficiency of UV LED before thermal effects take over. The constant external quantum efficiency of the blue or UV LEDs at high current densities rule out the carrier saturation effect unambiguously. It is also experimentally shown that increasing the number of quantum wells from 5 to 10 is found to be effective in the utilization of injected carriers for radiative recombination and hence in improving the luminescence efficiency of the UV LEDs. Second, the methods of low defect density buffer layer growth, carrier confinement improvement in the quantum wells and p-type AlxGa1-xN cladding layer are introduced for further improving the output power of UV LEDs. Etch pits density is improved by almost one order of magnitude with the use of three SiNx layers as an interlayer in the LED structure. It is also found that adding a little amount of Al into GaN to form a barrier layer does not degrade the crystal quality. Instead, it leads to better carrier confinement in shallower In0.06Ga0.94N quantum well. The luminescence intensity of the UV LED structure with Al0.05Ga0.95N barrier layers is about four-fold higher than that of its counterpart, revealing that the carrier confinement ability and material quality both are significantly improved by introducing a little Al and SiNx interlayers into barrier layers and the n-GaN underlying buffer layer, respectively. The emission intensity of 400 nm UV LED with a graded p-AlxGa1-xN layer is also greatly improved, because of the effective increase in the hole injection effect from p-GaN to the quantum well layers and the increase in the recombination emission rate of electron and hole pairs. Third, the effects of etching depth and stripe orientation on the structural and optical properties of the GaN epilayer as well as LEDs are investigated. It is found that much better material quality and light output power are obtained when GaN epilayer and LEDs are grown on a 0.9 μm-deep patterned sapphire substrate with stripes along the <1-100>sapphire direction. According to the results of scanning electron microscopy, transmission electron microscopy (TEM), and cathodoluminescence (CL) studies on these samples, the growth modes of GaN epilayer along the <11-20>sapphire and <1-100>sapphire directions are proposed and used to explain the anisotropic material properties between them. Normalized far field emission patterns of the LEDs show that the emitted light are scattered predominantly toward the vertical direction with increasing the groove depth. Their radiation angles, however, are stripe-orientation independent, implying that the light extraction efficiency is almost the same for the LEDs grown on PSSs with stripes along the <11-20>sapphire and <1-100>sapphire directions. Otherwise, the analysis of the internal quantum efficiency (IQE) and light extraction efficiency (LEE) in these two sets of UV LEDs was also investigated. It was also found that the improvement in the output power was mainly due to the enhancement of the LEE and then been changed to the contribution of the IQE by reducing the dislocation density when the LED structure was grown on PSS with stripes along the <11-20>sapphire and <1-100>sapphire directions, respectively. Finally, we can conclude that the maximum contribution of the LEE was about 44.9 % obtained using the 0.9 μm-deep stripe-PSS along either the <11-20>sapphire or <1-100>sapphire direction. However, an improvement of the IQE by as much as 2.785 times higher can be obtained for a LED structure grown on 0.9 μm-deep stripe-PSS along the <1-100>sapphire direction than its counterpart. It is therefore concluded that the superior performance of LED on the <1-100>sapphire stripe is attributed to its lower defect density. Finally, sapphire substrates with stripe SiON patterns which has adjustable refractive index to match sapphire substrate have been shown effective in improving the output power of light-emitting diodes. The output power of 400 nm UV LEDs grown on the stripe-SIONPSS is increased as much as 60 % compared to the LEDs grown on planar one. In the other work, the estimated values of the IQE and LEE in these LED grown on stripe-SIONPSS and planar substrate are obtained by temperature-dependent PL measurement. The IQE and LEE in the LED grown on stripe-SIONPSS are 28.6 % and 37.62 %, respectively. The increments of the IQE and LEE are about 26.55 % and 26.9 %, respectively, comparing with that of its counterpart. It shows that the power output enhancement of our devices is due not only to the reduction of dislocation density but also to the light extraction contributed by the geometric shape of the SiON stripe patterns. These results indicate that SiON stripe pattern on sapphire substrate is a promising approach to achieve high efficiency LEDs. |
